Instruments for measuring particles and/or separating particles are utilized in an increasing number of situations. Perhaps the single largest field of use is in the semiconductor industry wherein cleanliness in fabricating semi-conductors is extremely important. For example, particle counters are utilized to monitor the number of particles in the so called clean rooms in which semiconductor manufacturing operations are conducted. Clean rooms are usually classified on the basis of the maximum number of particles of a given size permitted per volume of air. To meet specifications and maintain quality levels of the product being made, it is necessary to ensure that particle contamination is kept to an acceptable minimum requirement.
The basic system utilized in most airborne particle counters transmits a sample air stream through a beam of light, which results in light energy being scattered by the particles. This energy is detected and measured by suitable optics and electronics. Significant improvements have been made in the optics and electronics employed for detecting and processing the scattered light signals. Also, the advent of lasers has greatly improved the quality and intensity of the illuminating light beam.
In spite of these improvements, the accuracy of the data output from the instruments still leaves much to be desired. Tests also reveal other inaccuracies, such as recirculating particles and appreciable discrepancies in absolute count correlation between different instruments.
Others have addressed this particle detection problem by developing particle measuring systems, wherein the particles to be detected or measured are entrained in a fluid stream which is passed through a light beam, typically a laser light beam. As stated earlier, the particles passing through the beam will scatter light which is collected and focused on a photodetector or photodetectors resulting in electrical pulses being generated. The intensity of the scattered light and, accordingly, the amplitude of the pulses generated by the photo-detectors provide an indication of the particle sizes.
In many systems, it is important to detect every particle in the entrained fluid stream. This need requires that the light beam pass through the entire cross sectional area of the fluid stream entraining the particles. When the fluid stream is a liquid stream, the conduit carrying the stream through the light beam is typically narrowed down to a small cross sectional area so that a high intensity beam can be caused to pass through the entire cross sectional area of the fluid stream. When the fluid stream entraining the particles is a gas stream of substantial density, the seeded gas stream is shaped by a nozzle into the form of the sheet and a laser beam is passed through the sheet-shaped stream along its long dimension as described, for example, in U.S. Pat. No. 4,746,215 to Kenneth P. Gross. The nozzle in this device has a slowly varying cross-section that does not appreciably affect the ability of the suspended particles to closely follow the gas streamlines (i.e., no separation occurs between particles and the gas phase). However, when the particles are in vacuum, as in semiconductor processing equipment, nozzles cannot readily be used to shape the stream into a sheet-like form. In addition, in order for the vacuum line to efficiently transmit a vacuum to the work area of the semiconductor processing, it is necessary for the vacuum line to have a substantial cross sectional area. Thus, the prior art particle detecting systems involving narrow conduits or nozzles for shaping the fluid stream have limitations which make them not fully satisfactory in detecting particles in a vacuum line such as that employed in semiconductor processing equipment.
In the manufacture of semiconductor and packaging devices using thin-film technology, one of the major yield detractors is defects caused by particle contamination on the thin-film surfaces. It is now known that a major proportion of such defects are caused by particle contamination generated within the process equipment or tools used to manufacture the thin-film devices, as disclosed by Bowling, R. A., et al., "Status and needs of in-situ real-time process particle detection," J. Environ. Sci., Vol. 32(5), pages 22-27 (1989).
Much of the process equipment used in thin-film manufacture or in the semiconductor industry operates at low pressures (e.g. RIE, PECVD). For these tools, in-situ particle monitors are more frequently being used to detect and control process-equipment related contamination, and are often mounted in the process chamber exhaust line.
Presently, available vacuum line monitors generally use light scattering to detect gas-borne particles flowing through a passage, a section of which is illuminated by a laser beam perpendicular to the flow direction as discussed in Greenstein, D., et al., "Investigating a Prototype In Situ Particle Monitor on the Exhaust Line of a CVD Reactor," Microcontamination, Vol. 3, pages 21-26 (1991) and Stern, J. E., et al., "Monitoring Downstream Particles in a Single-Wafer CVD Oxide Reactor," Microcontamination, Vol. 11, pages 17-56 (1991). In these instruments, a single focused laser beam is used to illuminate a small portion of the flow passage cross-section. Only a very small fraction of the particles in the flow are illuminated and thus detected. The low detection efficiency (on the order of 0.1-3.0 percent) of such particle monitors often results in poor quality particle count statistics, thus making the monitor less useful for statistical process control (SPC) applications.
There have been a number of attempts to resolve the problem of low detection efficiency. In one case a multiply reflected beam was used to create a light "sheet" or "net" which increased the illuminated area in the vacuum conduit, as disclosed by Borden, U.S. Pat. No. 4,739,177. However, an independent study by Caldow, R., et al., "Performance of the High Yield Technology Inc. PM-100 In Situ Particle Flux Monitor," Aerosol Science Technology, Vol. 12, pages 981-991 (1990), noted only a modest increase in efficiency of a system, such as the one disclosed by Borden. Their results showed a 4.7 percent monitor counting efficiency for 6.0 micron particle size, while the efficiency for the 0.5 micron particle size was only 0.1 percent, when a multiply reflected beam system made by High Yield Technology, the assignee of Borden's U.S. Pat. No. 4,739,177, was used.
A more recent device documented in U.S. Pat. No. 5,092,675 to Sommer, uses a sheet of laser light to illuminate the entire cross-section area of the vacuum conduit, by which means every particles flowing in the conduit may be detected. This increase in the illuminated cross-section may, however, require lasers of correspondingly higher power to maintain the minimum detectable particle size at current levels.
Thus, further improvements in particle detectors or counters is needed.
The invention disclosed herein also detects particles by light scattering. However, the novel design utilizes a specially designed system that uses the light source, beam optics, photodetector elements and a unique nozzle design that aerodynamically focuses most of the suspended particles in the fluid flow into a focal region whose cross-section is small in comparison to the nozzle exit area. The focal region is illuminated by one or more narrow, intense beams of light, preferably a focused laser beam, and thus a large fraction (estimated to be close to 100 percent) of the particles in the flow can be detected. The high particle detection efficiency that can be obtained with this new invention is well suited for many industrial applications, such as, the SPC applications.